What is Reactive Strength?

What is Reactive Strength?

Reactive strength is the quality that lets you catch a fall and turn it into upward or forward motion almost instantly, the difference between an athlete who lands from a jump and looks like they bounced off concrete versus one who sinks in and grinds back up. It sits at the fast end of the stretch-shortening cycle (SSC), the eccentric-then-concentric muscle action that happens every time you land, cut, or push off the ground.

You use it constantly in sport: the touch-down phase of a sprint stride, the plant step before a change of direction, the second jump in a rebound layup, the takeoff after a hurdle. Sport scientists measure reactive strength with the reactive strength index (RSI), most commonly from a drop jump: you step off a box, land, and rebound into a maximal vertical jump while a force plate or contact mat times how long your feet stay on the ground.

RSI is simply jump height divided by ground contact time, so a bigger number means you produce more height per unit of ground time, exactly what a fast SSC action demands. Reactive strength is distinct from maximal strength and from raw jump height. A powerlifter can squat triple bodyweight and still post a modest RSI if their feet linger on the ground; a lighter, springier athlete with less absolute strength can post a far higher RSI because they get off the floor fast.

How it works

Reactive strength works through the three phases of the stretch-shortening cycle. The eccentric phase happens the instant your foot contacts the ground: the calf, quad, and glute muscles lengthen under load while tendons like the Achilles and patellar tendon stretch and store elastic energy, the way a stretched rubber band stores energy. The amortization phase is the brief transition between the stretch and the push-off; the shorter this pause, the more of that stored elastic energy survives to be reused rather than dissipated as heat.

The concentric phase is the push-off itself, when the tendon recoils and the muscle actively shortens, releasing the stored energy plus new contractile force into the jump, stride, or cut. A fast SSC action, the kind reactive strength describes, keeps ground contact under roughly 250 milliseconds, as seen in sprinting, hopping, and drop jumps; a slow SSC action, like a countermovement jump with a deep knee bend, takes longer and relies more on active muscle force than stored elastic recoil.

Elite sprinters spend only about 80 to 110 milliseconds on the ground at top speed, which is one reason reactive strength correlates so strongly with sprint performance: the faster you complete that stretch-reflex loop, the less time gravity and ground contact steal from your stride. The calf-ankle complex dominates short-contact actions because the Achilles tendon is remarkably stiff and springy, while longer-contact actions recruit more knee and hip extension. Nervous-system factors matter too: a quicker stretch reflex and greater tendon stiffness both shorten amortization and raise RSI.

The formula

RSI = Jump height ÷ Ground contact time

Example: a 0.36 m jump with a 0.18 s ground contact gives an RSI of 2.0. You raise RSI by jumping higher, cutting contact time, or both, but cutting contact time without losing height is usually the bigger lever for sport transfer.

How to apply it

• Drop jumps for testing and training: Step off a box 20 to 40 cm high, land, and rebound as fast as possible rather than as high as possible. Because the instruction is to minimize ground contact, the drop jump trains the fast SSC directly and is the standard way to calculate RSI.
• Depth jumps for explosive strength: A similar setup to a drop jump, but the cue is to jump as high as possible after landing, which allows a longer, more forceful ground contact. Depth jumps build explosive strength and complement drop jumps, which sharpen pure reactive speed.
• Hurdle hops: Clear 5 to 10 low hurdles with quick, repeated two-foot hops, keeping ground contact under about 200 milliseconds and the knees mostly stiff. This drills rhythm, ankle stiffness, and the ability to reuse elastic energy hop after hop.
• Pogo jumps: Small, ankle-dominant hops with almost no knee bend and still arms. Pogos isolate the calf-ankle complex and Achilles tendon stiffness that short-contact reactive strength depends on, and they double as a low-intensity entry point into plyometrics.
• Bounding and single-leg bounds: Repeated long, horizontal jumps, alternating legs or hopping on one leg, that apply reactive strength to sprinting and change-of-direction mechanics rather than pure vertical output. Horizontal bounds transfer more directly to field-sport acceleration than vertical drop jumps do.
• Find the optimal box height: Test RSI across a range of box heights, commonly 20, 30, 40, and 50 cm, and train at the height that produces your highest RSI, not the tallest box you can survive. RSI typically rises then falls as height climbs past an individual's optimum.

Types

Worked example

An incremental drop jump test finds the box height that produces the best reactive strength for one athlete. Same athlete, four box heights, jump height and ground contact time measured by a contact mat at each.

RSI climbs from 20 to 30 cm as the athlete gets marginally higher without losing speed off the ground, plateaus at 40 cm, then drops at 50 cm because contact time balloons faster than jump height can compensate. This athlete's optimal training height sits at 30 to 40 cm, not the tallest box available.

Reactive strength vs maximal strength

The two qualities feed each other. Research on plyometric readiness recommends a strength base, commonly a free-weight squat of about 1.5 to 2.5 times body mass, before loading up on high-intensity depth jumps, because that floor lets tendons and joints tolerate the landing forces reactive-strength training creates.

By goal

• Sprinters and jump-sport athletes: Prioritize horizontal bounds and low-box drop jumps that keep ground contact under 200 ms, and retest RSI every 4 to 6 weeks at a fixed box height to track the fast-SSC quality that most directly predicts top-speed sprinting and jump performance.
• Team-sport and field athletes: Mix vertical drop jumps with hurdle hops and single-leg bounds so reactive strength transfers to cutting and rebounding, not just jumping straight up. Two reactive-focused sessions a week, 20 to 40 quality ground contacts each, is a common starting volume.
• Beginners and general strength trainees: Build a strength base first. Many coaches use a free-weight squat around 1.5 times body mass as a readiness marker, then start with pogo jumps and low-box drop jumps before progressing to true depth jumps or high hurdles.

Common misconceptions

• "Reactive strength is just how high you can jump." Jump height is only half the ratio. Two athletes can jump the same height, but the one who does it with a shorter ground contact has a higher RSI and more usable reactive strength for sprinting and cutting, where the ground only gives you a fraction of a second.
• "A taller drop-jump box always builds more reactive strength." Research on drop-jump technique shows RSI often falls once box height passes an individual's optimum, because ground contact time increases faster than jump height does. The best box height is the one that maximizes RSI for that athlete, not the tallest one available.
• "RSI and RSImod measure the same thing and can be swapped." Studies comparing the two report only poor agreement between drop-jump RSI and countermovement-style RSImod scores, even though both correlate with performance. Track whichever version you test first and stay consistent rather than mixing the two across sessions.
• "You need to be an elite athlete before touching plyometrics." Low-intensity drills like pogo jumps and small-box drop jumps are appropriate early, once basic landing mechanics are sound. Higher-intensity depth jumps are what typically wait for a strength base, such as a squat around 1.5 times body mass.

Related terms

Reactive Strength FAQ

What is reactive strength in simple terms?

Reactive strength is how well you catch the force of landing and fire it straight back out into a jump, sprint stride, or cut, instead of absorbing it and pausing. Coaches score it with the reactive strength index (RSI): jump height divided by ground contact time.

What is a good RSI score?

For a standard drop jump, an RSI around 2.0 is a solid benchmark for most trained athletes, 2.5 and above is excellent, and elite sprinters and jumpers often exceed 3.0. Below 1.5 usually signals reactive strength is an underdeveloped quality worth training directly.

How is reactive strength index calculated?

RSI equals jump height divided by ground contact time, both measured during a drop jump off a box using a force plate or contact mat. For example, a 0.36 m jump with a 0.18 s ground contact gives an RSI of 2.0.

What is the difference between RSI and RSImod?

RSI comes from a drop jump and divides jump height by ground contact time. RSImod comes from a countermovement-style jump and divides jump height by total time to takeoff. The two correlate, but studies show poor agreement, so they are not interchangeable.

What is the best exercise to build reactive strength?

Drop jumps off a 20 to 40 cm box, performed for fast ground contact rather than maximum height, are the standard reactive-strength builder. Hurdle hops, pogo jumps, and bounding add variety and train the quality in horizontal and single-leg patterns too.

How does reactive strength relate to sprinting?

Elite sprinters spend only about 80 to 110 milliseconds on the ground at top speed, a fast stretch-shortening cycle action. Higher reactive strength means you complete that landing-to-push-off loop faster, which is why RSI correlates strongly with sprint speed and acceleration.

Is reactive strength the same as explosive strength?

No. Explosive strength is generating maximal force quickly from a static or slow start, like a vertical jump from a deep squat. Reactive strength specifically involves absorbing an impact first, so it depends on tendon stiffness and stretch-reflex speed that pure explosive strength does not.

How often should I train reactive strength?

Two focused sessions a week is a common starting point, using low volumes of 20 to 40 quality ground contacts per session with full recovery between reps. Lower-intensity work like pogo jumps can be layered in as a warm-up on other days.

Do I need to be strong before doing plyometrics?

Low-intensity drills like pogo jumps are fine early on. Higher-intensity depth jumps are usually reserved for athletes who can free-weight squat about 1.5 to 2.5 times their body mass, since that strength floor helps the tendons and joints tolerate landing forces safely.

Can reactive strength be improved, and how fast?

Yes. A 2022 systematic review found plyometric training produces a large, statistically significant improvement in RSI in both adult and youth athletes over typical 6 to 10 week programs, making it one of the more trainable qualities in strength and conditioning.

References

1. Struzik A, et al. Effect of drop jump technique on the reactive strength index. Journal of Human Kinetics, 2016
2. Sole CJ, et al. Preliminary Scale of Reference Values for Evaluating Reactive Strength Index-Modified in Male and Female NCAA Division I Athletes. Sports (Basel), 2018
3. Rebelo A, et al. How to Improve the Reactive Strength Index among Male Athletes? A Systematic Review with Meta-Analysis. Healthcare (Basel), 2022
4. Louder T, et al. Association and Agreement between Reactive Strength Index and Reactive Strength Index-Modified Scores. Sports (Basel), 2021
5. Davies G, et al. Current Concepts of Plyometric Exercise. International Journal of Sports Physical Therapy, 2015
6. Pleša J, et al. The association between reactive strength index and reactive strength index modified with approach jump performance. PLoS One, 2022
7. Stretch-shortening cycle. Wikipedia